Initial studies that were performed on pancreatic islets were candidate-driven and indicated a correlation between lower mRNA expression and excessive promoter hypermethylation of INS (encoding insulin), PDX1 (encoding a transcription factor that is important for both pancreatic development and the function of mature β-cells), GLP1R (encoding the GLP1 receptor that stimulates insulin secretion), and PPARGC1A (encoding the mitochondrial regulator peroxisome proliferator-activated receptor γ coactivator 1α [PGC1α]) in islets from T2D donors [55]. Here, INS is linked to type 2 diabetes mellitus.